What we looked at
Mid-priced steel flask, screw lid
Lost 8°C over twelve hours, sealed reliably in a bag on its side, and the lid comes apart for cleaning. Around £25 and the one we would buy.
RecommendedPremium steel flask, push-button lid
Best insulation in the test at 6°C lost, and the push-button mechanism cannot be disassembled, so it develops a smell within a month.
Recommended with reservationsBudget steel flask, screw lid
Lost 12°C over twelve hours, which is still drinkable coffee. Seal held. At around £15 it is the honest value pick if you do not mind the extra drop.
Recommended with reservationsFlip-top lid designs, as a category
All three flip-top flasks in this test leaked when laid on their side in a bag. The convenience is real and the failure mode is your laptop.
Not recommendedWe filled six flasks with water at 96°C, sealed them, left them on a kitchen counter overnight, and measured the temperature twelve hours later. Then we put each one on its side in a bag with a tea towel to see what came out.
The insulation results were boring
| Flask | Start | After 12 h | Lost |
|---|---|---|---|
| Premium, push-button lid | 96°C | 90°C | 6°C |
| Mid-priced, screw lid | 96°C | 88°C | 8°C |
| Mid-priced, flip-top | 96°C | 87°C | 9°C |
| Budget, screw lid | 96°C | 84°C | 12°C |
| Budget, flip-top | 96°C | 83°C | 13°C |
| Novelty branded, flip-top | 96°C | 81°C | 15°C |
A 9°C spread across a threefold price range. Every one of them would still be unpleasantly hot to sip immediately at the twelve-hour mark.
If your requirement is “coffee still hot at lunchtime”, all six pass and you should buy on price.
The lid is where they actually differ
Three of the six leaked when laid on their side.
All three were flip-top designs. The mechanism that makes them convenient — a hinged spout you open with a thumb — is also a seal that relies on a small gasket under spring pressure, and on three of these it did not hold horizontally.
A flask that leaks in a bag has failed completely, however well it insulates. That is not a minor deduction; it is a different category of product.
The thing nobody tests: can you clean it
The premium flask had the best insulation in the test and we would not buy it again.
Its push-button lid cannot be taken apart. After about a month of daily coffee there is a smell coming from inside a mechanism you cannot reach, and no amount of rinsing fixes it. We have replaced two of these over the years for exactly this reason and each time blamed ourselves.
The mid-priced screw lid comes apart into three pieces, all of which go in a dishwasher. It lost 2°C more overnight and it will still be in use in five years.
What we would buy
The mid-priced screw-lid flask, around £25. Second-best insulation, a seal that held on its side, and a lid that disassembles.
If money is tight, the budget screw-lid at around £15 is an honest purchase — 12°C lost over twelve hours is still hot coffee, and the seal held.
Avoid flip-tops unless you will only ever carry it upright. The convenience is real. So is the laptop.
The maintenance nobody does
Lid gaskets perish. They go hard, then they leak, and people conclude the flask is worn out and buy another one.
Two of the six manufacturers sell replacement gaskets for a couple of pounds. Those are the two whose flasks we still own from previous tests. Check for that before you check the insulation figures, because it is the difference between a five-year flask and an eighteen-month one.
Martin Halvorsen
Contributing writer, Household
Writes about the objects in a home and how long they last. Buys everything he writes about.
Elsewhere in the magazine
- Food storage containers: we used six sets for eighteen months and the lids decided everything
- Chef's knives: I cooked with five for a year and the £30 one is still the one I reach for
- Nonstick pans: I bought five and cooked eggs in them for two years. Price predicted nothing.
- LED bulbs: the ones that died early all died in the same two places